In the test and measurement field, it is often desirable to operate a plurality of swept electronic instruments synchronously. Such is the case, for example, when measuring the performance of a mixer that is excited with input signals at one frequency and produces output signals at a different frequency. To characterize the mixer performance over a range of frequencies, the input is swept and the output is measured at an offset frequency swept in synchrony with the input. To illustrate, if the input signal sweeps across a frequency range of 1.0 to 1.1 MHz in a period of one second and is mixed with a 20 MHz local oscillator signal, the output monitoring instrument should sweep across the range 21.0 to 21.1 MHz, all within the same 1 second time period.
A more challenging example is measurement of harmonics produced by an amplifier circuit. In this case, the amplifier input may be swept from 1.0 to 1.1 MHz by a first instrument and a plurality of other instruments may monitor the amplifier's harmonic output by sweeping across different harmonic bands in synchrony with the input. Thus, one of the other instruments may sweep from 2.0 to 2.2 MHz, another of the instruments may sweep from 3.0 to 3.3 MHz, etc.
In the prior art, such measurements have been difficult to perform due to the difficulty in obtaining the requisite synchronized sweeps. It will be recognized that if the excitation signal is exciting the amplifier in the above-noted harmonic analysis example with a 1.05 Mhz signal, while a monitoring instrument is listening for the second harmonic at 2.0999 MHz (instead of 2.1000 MHz), the resulting analysis will be flawed. The degree of the error depends on the width of the passband within which the monitoring instrument is listening for the amplifier response. If the passband is 100 hertz, a 100 hertz skew between the harmonic of the excitation signal and the frequency of the monitoring instrument will prevent the signal from being properly detected.
Although the task of synchronizing the sweeping operation of several instruments may seem simple, it is not. The often-tried solution is to share among all the instruments a common reference frequency signal and common triggering signals. However, this solution fails because the instruments do not start their swept operations immediately in response to the triggering signal. Instead, there is a latency period, and the latency period may be different between identical instruments, even those sharing a common reference frequency signal. Consequently, instruments triggered at the same instant do not necessarily begin their swept operations simultaneously.
The variable latency period is attributable to the fact that the swept operation cannot be begun until certain clock signals internal to each instrument reach predetermined phase conditions. For example, in a representative instrument, internal clocking signals of 250, 187.5 and 100 KHz are used and the instrument cannot begin a sweep until all of the clocking signals share a common edge transition. This occurs once every 80 microseconds (i.e. once every 20 cycles of the 250 KHz signal, once every 15 cycles of the 187.5 KHz signal and once every 8 cycles of the 100 kilohertz signal). Even if these internal clocking signals are derived from the shared reference frequency signal (i.e. by frequency dividers), the phase relationships among the signals may be different in different instruments. This is because the phase, for example, of a 250 KHz signal derived from a 10 MHz reference signal in one instrument may have one of forty different phase relationships with a 250 kilohertz signal derived similarly in another instrument, depending on which cycle of the 10 MHz signal each instrument's divider circuitry began its operation. Other clocking signals in the instruments may be similarly skewed.
Accordingly, it is an object of the present invention to overcome this variable latency problem.
It is a more particular object of the present invention to sweep two or more instruments synchronously, with any desired mapping from one swept frequency range to the others.
It is another more particular object of the present invention to permit synchronized sweeps without requiring a handshaking operation between instruments.
In accordance with the present invention, the clocks in each of the instruments are reset and are started at the same instant by an externally applied signal The instruments' internal clock signal generators thus all begin simultaneously at a known phase and thereafter operate in tandem. When a trigger is thereafter received, there is still a latency period before a swept operation begins. However, since the clocks in all the instruments are operating synchronously, the latency period is the same for all the instruments.
The foregoing and additional features and advantages of the present invention will be more readily apparent from the following detailed description thereof, which proceeds with reference to the accompanying drawings.